Background of the Invention
This invention relates generally to the production of an activated sorbent for use in the removal of gaseous pollutants from a gas stream. In one embodiment, this invention relates to methods and apparatuses for the on-site production of an activated sorbent, such as activated carbon, in combination with the addition of certain chemicals that enhance the activity or effectiveness of the activated sorbent to produce a chemically-enhanced activated sorbent. Further, this invention relates to methods and apparatuses for the use of the chemically-enhanced activated sorbent in the removal of gaseous pollutants from a gas stream. In another embodiment, this invention relates to methods and apparatuses for the addition of certain chemicals that convert certain gaseous pollutants in a gas stream to a form that is more easily removed from the gas stream, for example, by a sorbent, including a chemically-enhanced activated sorbent.
Description of Related Art
Controlling emissions of certain gaseous pollutants, such acid gases and air toxics, in flue gases generated from combustion devices, such as coal-fired power plants, is an important concern. In such flue gases, certain air toxics and other species can be distributed in both the vapor phase and the solid phase, thereby requiring the removal of both solid matter, such as particulate matter, as well as vapor phase species. Further, certain air toxics (e.g., mercury), certain acid gases (e.g., SO3, HCl), and other air toxics (e.g., arsenic, selenium, nickel, lead, organics, etc.) that are present in the vapor phase of such gases are typically found in very low concentrations making removal difficult.
Sorbents are sometimes used to remove certain pollutants from gas streams. For example, sorbents may be injected into a gas stream and collected downstream after adsorbing a targeted vapor phase contaminant. Alternatively, a packed bed, an entrained bed, or fluidized bed may use a sorbent to adsorb a given vapor phase pollutant from a gas stream.
Calcium-based sorbents such as limestone, dolomite, lime, hydrated lime, calcium acetate, and calcium magnesium acetate are used, for example, in the removal of sulfur-based vaporous pollutants, such as SO2 and SO3, from coal-fired power plant flue gases. These sorbents can also be effective in removing other acid gases, such as HCl and H2S, as well as vaporous air toxic species including metal compounds such as selenium, arsenic, nickel, lead, and others found in trace amounts in coal-fired power plant flue gases. However, the process by which hydrated lime or lime, in particular, is produced is energy intensive, making these sorbents relatively expensive.
Regarding the removal of mercury, activated carbon is a sorbent used for sorption of mercury species from coal combustion flue gases and other industrial gas streams. The results from pilot and full-scale field demonstration tests have revealed that at a carbon loading of less than 5 lb/MMft3, mercury removal greater than 90% is achievable for flue gases generated from burning low-sulfur coals. However, mercury removal efficiency for flue gases generated from burning high-sulfur coals, which commonly contain greater than 10 ppm SO3, a level significantly higher than that in flue gases generated from lower sulfur coals, have been reported to be less than 50% at comparable activated carbon loadings.
In view of the foregoing, there is a need for an improved, low-cost method for generating a sorbent for use in removing certain gaseous pollutants from gas streams. Further, there is a need for generating a sorbent that has sufficient activity or effectiveness in adsorbing such gaseous pollutants.